Thermocouple structure and method of making same

ABSTRACT

A length of metallic wire is positioned inside a length of metallic tube in approximate axial alignment with the tube and set in from one end of the tube. The open area at the one end of the tube is reduced prior to heat sealing to facilitate the seal formation. The reduction is accomplished by setting a pellet on the end of the wire, by swaging the one end of the tube onto the adjacent portion of the wire, by placing a short tight-fitting sleeve around the end of the wire, or by upsetting the end of the wire. The sleeve can be the same or a different metal from the tube.

ted States Patent Wagner Nov. 27, 1973 THERMOCOUPLE STRUCTURE AND METHOD OF MAKING SAME Related US. Application Data Continuation-in-part of Ser. No. 82762, Oct. 21, 1970, Pat. No. 3,741,816, which is a continuation-impart of Ser. No. 734,512, June 3, 1968, Pat. No. 3,556,864.

References Cited UNITED STATES PATENTS l/1971 Wagner 29/573 Mellor et a1. 136/228 Qoulter et al. 29/482 X Primary Examiner-Charles W. Lanham Assistant Examiner-W. C. Tupman Attorney-Christie, Parker & Hale [57] ABSTRACT A length of metallic wire is positioned inside a length of metallic tube in approximate axial alignment with the tube and set in from one end of the tube. The open area at the one end of the tube is reduced prior to heat sealing to facilitate the seal formation. The reduction is accomplished by setting a pellet on the end of the wire, by swaging the one end of the tube onto the adjacent portion of the wire, by placing a short tight-fitting sleeve around the end of the wire, or by upsetting the end of the wire. The sleeve can be the same or a different metal from the tube.

19 Claims, 12 Drawing Figures Patented Nov. 27, 1973 2 Sheets-Sheet 2 TI-IERMOCOUPLE STRUCTURE AND METHOD OF MAKING SAME CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of my copending U.S. Pat. application, Ser. No. 82,762, filed on Oct. 21, 1970, now US. Pat. No. 3,741,816, which is a continuation-in-part of my U.S. Pat. No. 3,556,864 which issued on Jan. 19, 1971 and was filed on June 3, 1968.

BACKGROUND OF THE INVENTION This invention relates to thermocouples and, more particularly, to thermocouple structures especially suited for use in the pilot burner of a gas heater and to a method for making such thermocouple structures.

Conventionally, the thermocouple used in a targettype pilot burner, of which the pilot burner disclosed in US. Pat. No. 3,291,185 issued to Harold A. McIntosh et al., is typical, comprises a length of wire and a length of tube made from dissimilar metals. The length of wire, which has an appreciably smaller diameter than the inside diameter of the tube, lies inside the tube in axial alignment therewith. Preferably, the extremity of the tube against which the flame of the pilot burner impinges has a smaller diameter and wall thickness than the base of the tube so as to promote large heat transfer radially inward through the extremity and small heat transfer axially from the extremity to the base of the tube. A tip is formed on the end of the extremity of the tube to seal its interior from the atmosphere and to form a hot thermocouple junction between the extremity of the tube and the adjacent end of the wire. A pair of leads, permanently joined respectively to the base of the tube and the adjacent end of the wire, connect the thermocouple to an electrical measuring instrument, or electrical coil, operating a control device.

The tube is made from a nonoxidizable metal such as stainless steel, but the wire is generally made from a metal that may be oxidizable. If the seal at the extremity of the tube is imperfectly formed by the tip, oxidation of the wire takes place more rapidly and the hot thermocouple junction deteriorates. The diameter of the wire is appreciably smaller than the inside diameter of the tube to avoid the possibility of a short circuit through contact between the wire and the tube at a point other than the hot junction. As a result, it is difficult to center the end of the wire sufficiently in the open area at the end of the extremity of the tube while the tip is being welded. Eccentricity between the end of the tube and the wire tends to cause an uneven distribution of the metals in the metallic weld bead forming the tip. As a result, stringers of the wire material may be produced in the tip that are eventually oxidized, leaving the tip porous. If more heat is used to form the tip in an attempt to eliminate imperfect seals, the danger exists that too much admixture of the dissimilar metals occurs at the tip to, form an efficient hot junction. The problem of imperfect seals at the tip becomes especially acute if the tip forming operation is automated by use of heliarc equipment.

SUMMARY OF THE INVENTION The invention involves the formation of a sealed tip at the end of a thermocouple tube by reducing the open area at the end of the tube prior to the application of heat to form the tip. As a result, the weld head can more easily bridge the gap at the end of the tube and a more reliable seal is formed, whether the hot thermocouple junction is formed at the tip or elsewhere.

In one embodiment, the open area is reduced by setting a pellet on the end of the wire. The pellet is larger in cross-section than the wire and is made from a metallic composition that is compatible with the tube. Accordingly, the pellet fuses with the weld bead and the tube and forms part of the sealed tip.

In another embodiment, the end of the tube is swaged onto the adjacent surface of the wire and a hot thermocouple junction is formed therebetween. In addition, the open area can further be reduced by setting a pellet on the end of the wire, as in the first embodiment, or by bending or swaging in the ends of the tube that extend beyond the end of the wire.

In another embodiment, a short, tight-fitting sleeve is placed over the end of the wire to bridge the gap between the wire and the end of the tube. The sleeve can be open or closed at its end and can be made from the same or a different but compatible metal from the tube. This metal must be such that it will generate a voltage when in contact with the wire, when heated. The hot thermocouple junction is formed between the adjacent surfaces of the sleeve and the wire. A feature of this embodiment is the use of a different metallic composition for the sleeve than for the tube. Accordingly, there can be selected for the sleeve a metallic composition that is the most effective thermoelectric generator with the metal of the wire, even though such metal may be less resistant to oxidation than the tube.

In another embodiment, the open area at the end of the tube is reduced by upsetting the adjacent end of the wire. The hot thermocouple junction is formed between the upset end of the wire and the sealed tip of the tube.

BRIEF DESCRIPTION OF THE DRAWINGS The features of specific embodiments of the invention are illustrated in the drawings, in which:

FIG. 11 is a side elevation view in section of a thermocouple structure of the type in which the invention is used;

FIGS. 2A and 2B are side elevation views in section of one embodiment of the extremity end of a tube respectively before and after the formation of the tip;

FIGS. 3A and 3B are side elevation views in section of another embodiment of the extremity end of tubes before the formation of the tip, and FIG. 3C is a side elevation view in section of the same tubes after the formation of the tip;

FIGS. 4A and 41B are side elevation views in section of a further embodiment of the extremity end of tubes before the formation of the tip, and FIGS. 4C and 4D are side elevation views in section of FIGS. 4A and 413 after the formation of the tip, respectively; and

FIGS. 5A and 5B are side elevation views in section of another embodiment of the extremity end of a tube, respectively, before and after the formation of the tip.

DESCRIPTION OF SPECIFIC EMBODIMENTS In FIG. l, a thermocouple structure 1, a connector 2, and a coaxial conduit 3 are shown. Thermocouple structure 1 comprises a length of tube 4 made from a nonoxidizable metal such as, for example, Type 446 stainless steel and a length of wire 5 made from a dis similar metal such as, for example, a 40-percent-nickel- 60-percent-copper alloy, copel. The term metal as used in this specification means a material that is a good conductor of electricity and the term dissimilar metals designates materials that exhibit the Peltier effect when brought into contact with one another. Tube 4 is centered on a longitudinal axis 8. The length of tube 4 has two regions, namely, a base 6 and an extremity 7. As depicted in FIG. 1, the wall thickness and diameter of extremity 7 are significantly smaller than the same dimensions of base 6. The purpose of this is to promote large heat transfer radially inward through extremity 7, i.e., in a direction perpendicular to axis 8, and small heat transfer axially from extremity 7 to base 6, i.e., parallel to axis 8. Wire is in approximately axial alignment with tube 4 along axis 8, and wire 5 has an appreciably smaller diameter than the inside diameter of tube 4. In other words, the gauge of wire 5 is so selected and wire 5 is so positioned within tube 4 that no contact takes place between the surface of wire 5 and the interior surface of tube 4 (except for a hot thermocouple junction) in spite of the tolerance variations that may occur in tube 4 and wire 5 and in spite of the variances in manufacture and of the forces to which thermocouple structure 1 may be subjected. One end of wire 5 is set in from the adjacent end of extremity 7 as depicted in FIG. 1. A hot thermocouple junction 9 is formed between a portion of extremity 7 and the adjacent portion of wire 5. Hot junction 9, which preferably covers a cylindrical area between extremity 7 and wire 5, could be formed by spot-welding techniques. In view of the appreciable difference required between the inside diameter of extremity 7 and the diameter of wire 5, it is preferable to reduce this difference in the area where hot junction 9 is to be formed prior to its formation. In the embodiment of FIG. 1, this is accomplished by upsetting the end of wire 5. The upset end portion 10 of wire 5 has a diameter almost as large as the inside diameter of extremity 7. A tip 11 is formed integral with the end of extremity 7 and is spaced from the end of upset portion 10.'Thus, tip 11 seals the end of tube 4 from the atmosphere without electrical contact with the set-in end of wire 5 or contamination of material of wire 10. Base portion 6 has annular grooves 12 and 13 for receiving a conventional clip used to mount thermocouple structure 1. Efficient operation of thermocouple structure 1 results as long as the flame of the pilot burner with which it is used impinges upon extremity 7, which gives substantial leeway in the position of thermocouple structure 1. In a conventional thermocouple with hot junction at the tip, the flame must impinge substantially on the tip for acceptable performance.

Connector 2 comprises a male member 20 that is permanently attached to thermocouple structure 1 and a female member 21 that is permanently attached to coaxial conduit 3. A sleeve portion 22 of male member 20, which is made from a dissimilar metal from tube 4, surrounds the end of base 6 and is fused thereto by spot-welding or brazing to form a cold thermocouple junction 23. One end of male member 20 has a flange for mounting purposes. The other end of male member has external threads 16 for mating with internal threads of female member 21. A disc-shaped terminal 24, which could be made, for example, from brass, is attached to the end of wire 5. This end of wire 5 and disc 24 may be silverplated for good conductivity. A M

button 25 made from a very good conductor such as 60-40 solder is located at the center of terminal 24 and joins the end of wire 5 mechanically and electrically to terminal 24. A disc-shaped electrical insulator 26 supports the end of wire 5 in spaced relationship from the interior of male member 20 through which wire 5 extends and centers wire 5 in relation to member 20 and portion 6 of tube 7. Thermocouple structure 1 and male member 20 cannot rotate with respect to each other. Female member 21 has a cavity 27 with walls on which internal threads are formed that mate with the threads of male member 20. Coaxial conduit 3, which is conventional and made from a metal such as, for example, copper, has an inner lead 28 and an outer lead 29 that are supported in spaced relationship by heatresistant insulation 35 on lead 28. Conduit 3 extends through an opening 30 in the back of female member 21 into cavity 27. The end of outer lead 29 within cavity 27 is flared so its diameter is larger than opening 30. Thus, the end of conduit 3 is permanently retained within female member 21. A disc-shaped terminal 31, which could be made, for example, from brass, is attached to the end of inner lead 28. A button 32, which is made from a good conductor such as 60-40 solder, is located at the center of terminal 31 and joins the end of inner lead 28 and terminal 31 mechanically and electrically. A cold thermocouple junction is also formed where wire 5 and inner lead 28 are connected by buttons 25 and 32. Although it is usually desirable in practice to utilize at least two cold thermocouple junctions as in the embodiment of FIG. 1, the invention can be practiced in a thermocouple employing a single cold junction. Inner lead 28 and terminal 31 are held in spaced relationship from outer lead 29 by a disc-shaped electrical insulator 33. The interior surface of female member 21 forming the transition between opening 30 and cavity 27 has an annular shoulder 34 that conforms to the flared end of outer lead 29. Female member 21 is capable of rotating with respect to the flared end of outer lead 29. When members 20 and 21 of connector 2 are mated as illustrated in FIG. 1, shoulder 34 bears against the flared end of outer lead 29 to form a good electrical contact therewith. As shoulder 34 bears against outer lead 29, terminal 31, which is spaced from outer lead 29 by insulator 33, is forced against terminal 24, thereby forming a good electrical contact between wire 5 and inner lead 28. In summary, one electrical path is formed from tube 4 through sleeve 22, male member 20, and female member 21 to outer lead 29. The other electrical path is formed from wire 5 through terminals 24 and 31 to inner lead 28. When thermocouple structure 1 is to be replaced, female member 21 is held and conduit 3 and male member 20 are rotated stationary until members 20 and 21 separate, thereby releasing thermocouple structure 1 from conduit 3.

In the construction of thermocouple structure 1, wire 5 is first positioned inside of tube 4 in axial alignment therewith such that its one end is set in from the end of extremity 7. Hot thermocouple junction 9 is then formed between a portion of wire 5 in the vicinity of the set-in end and the adjacent portion of extremity 7. Finally, the end of extremity 7 is sealed without forming an electrical contact with the set-in end of wire 5.

Reference is now made to FIGS. 2 through 5 for a discussion of techniques for improving the sealed tip formed at theend of the extremity of the tube of a thermocouple structure. The described techniques are for the most part applicable to the formation of tip 11 in the embodiment of FIG. 1, as well as to the formation of a concurrent seal that also forms the hot thermocouple junction. The general concept involved in these techniques is that the open area at the end of the tube is reduced prior to the welding operation that forms the seal.

In FIG. 2A are shown a tube 40 and a wire 41 that are made from dissimilar metals. The end of wire 41 is set in slightly from the end of tube 40 when it is placed therein. Then, the open area 42 at the end of tube 40 is reduced by setting a pellet 43 in the mouth of tube 40 on the end of wire 4|. Pellet 43 is larger in crosssection thanwire 41 and is made from a metal compatible with tube 40, preferably from the same metallic composition as tube 40. As used in this specification, the term compatible wit means that the two compatible metals are capable of being fused together to form an effective atmospherically sealed tip at the end of tube 40, e.g. the two metals are capable of being completely welded together. The pellet provides extra weld material of a compatible composition with tube 7, and also protects wire 41 from direct contact with the welding are, thereby minimizing alloying of wire material 41 in tip 44. FIG. 2B depicts the same components as FIG. 2A after the end of tube 40 is welded shut by application of heat. A sealed tip 44 is formed that provides a good seal from the atmosphere and an efficient hot thermocouple junction between tube 40 and wire II. To employ this technique in forming tip 11 in FIG. I, pellet 43 is spaced from the end of the wire by an insulator or by swaging a portion of the tube onto the pellet to retain it at the end of the tube in spaced relationship from the end of wire 41, or by using a pellet with a shoulder to hold it in place at the end of tube 4.

In FIG. 3A, a tube 50 and a wire 51 are shown. Wire EI is set in slightly from the end of tube 50 when it is placed therein. Then, a portion of tube 50 near its end is swaged onto the adjacent surface of wire 51 and a hot thermocouple junction is formed at the swaged region by spot welding. The swaging of tube 50 serves to reduce the open area 52 at the end of tube 50. This eliminates the problem of centering wire 51 within the interior of tube 50. Next, open area 52 may be further reduced by placing a pellet 53 made from a metal compatible with tube 50 in the mouth of tube 50 on the end of wire 51. In FIG. 3B the need for pellet 53 is eliminated by swaging or bending inward the portion of the length of tube 50 that extends beyond the end of wire 51 to form a conical region 54, after the hot thermocouple junction is formed. As illustrated in FIG. 3C, a sealed tip 55 is formed when the end of tube 50 in FIGS. 3A or 3B is welded shut by application of heat. Alternatively, the hot thermocouple junction could be formed at tip 55 during sealing, without pellet or swagmg.

In FIG. 4A, a tube 60 and a wire 61 are shown. A tight-fitting open ended sleeve 62 is place over the end of wire 61. Then wire 61 with sleeve 62 is placed in tube 60 such that the end of wire 61 is set in slightly from the end of tube 60 and tube 60 and sleeve 62 are approximately flush. The outside diameter of sleeve 62 is sufficiently smaller than the inside diameter at the end of tube 60 to fit loosely or tightly therein. Sleeve 62 is made from a metal that is compatible with tube 60. Sleeve 62 is short in relation to the length of tube 60 and wire 61. Before or after placing wire 61 and sleeve 62 in tube 60 and in an electrical generator when heated in contact with wire 61, sleeve 62 is spot-welded to wire 61 to form a hot thermocouple junction therebetween. At the same time, the spot-welding-may be used to establish a good electrical contact between tube 60 and sleeve 62 if the hot thermocouple junction is formed after placement in tube 60. In FIG. 413, a closed ended sleeve 63 replaces sleeve 62. Sleeve 63 is treated in the same manner in the course of the formation of a thermocouple as sleeve 62. It simply provides greater protection of the end of wire 61 from the heat during formation of the sealed tip and a more uniform tip weld bead. As illustrated in FIGS. 4C and 4D, sealed tips 64 and 65 are formed on the end of tube 60 when the end of tube 60 is welded shut by application of heat.

FIG. 4C illustrates open-ended sleeve 62 after the tip has been formed, and FIG. 4D illustrated close-ended sleeve 63 after the tip is formed.

Although sleeves 62 and 63 could be made from the same metallic composition as tube 60, special advantage can be derived from making them from different compatible compositions. In this way, there can be selected for the tube a composition which is highly resistant to oxidation and for the sleeve a composition which forms the most efficient thermo-electric generator with the metal of the wire. For example, the wire could be made from copel, the sleeve could be made from stainless steel 403, 'and tube 60 could be made from stainless steel 430. Stainless steel 403 is a better thermal generator with copel but is less resistant to oxidation than stainless steel 430. Another advantage can be derived from using a different metal for the sleeve, namely, that a precious metal can be used as one element of the thermocouple junction to improve the thermoelectric generation without undue expense. The sleeve requires much less metal than the tube.

In FIG. 5A, a tube and a wire 71 are shown. Prior to placing wire 71 in tube 70, its end is upset to form a portion 73. As illustrated in FIG. SE, a sealing tip 74 is formed when the end of tube 70 is welded shut by ap plication of heat. Tip 74 serves to seal the end of tube 70 and to form the hot thermocouple junction with the end of upset portion 73. This thermocouple has the advantage of providing high amperage because of the proximity of the hot thermocouple junction to the source of heat with which it is used. Further, it does not oxidize quickly because of the restricted passage at the end of tube 70 for the air and moisture entrapped inside tube 70 to reach the hot thermocouple junction.

In the embodiments of FIGS. 4A, 4B, 4C and 4D and FIGS. 5A and 5B, the effective cross-sectional area of the wire is increased, in the former case by a sleeve and in the latter case by an upset portion of the wire.

Of course, other ways of reducing the open area of the tube prior to the welding operation are also included within the scope of the invention. The described embodiments of the invention are only considered to be preferred and illustrative of the inventive concept; the scope of the invention is not to be restricted to such embodiments. Various and numerous other arrangements may be devised by one skilled in the art without departing from the spirit and scope of this invention.

What I claim is:

1. The method of maing a thermocouple comprising the steps of:

positioning a length of wire made from one metal inside a length of tube made from a dissimilar metal in approximate axial alignment with the tube and set in from one end of the tube, the tube having an inside diameter appreciably larger than the diameter of the wire;

reducing the open area at the one end of the tube after positioning the wire inside the tube; applying heat to the one end of the tube to seal the one end of the tube from the atmosphere at the reduced area so the seal forms a hot thermocouple junction between the tube and the wire; and forming a cold thermocouple junction including at least one of the lengths.

2. The method of claim 1, in which the open area at the one end of the tube is reduced by placing in the mouth of the tube on the set-in end of the wire a pleeet made from a metal compatible with the tube.

3. The method of claim 1, in which the open area at the one end of the tube is reduced by swaging a portion of the length of the tube at the one end onto the adjacent portion of the wire.

4. The method of claim 3, in which the open area at the one end of the tube is further reduced by placing in the mouth of the tube on the set-in end of the wire a pellet made from a metal compatible with the tube.

5. The method of claim 3, in which the open area at the one end of the tube is further reduced by bending in the portion of the length of tube extending beyond the set-in end of the wire at the one end of the tube.

6. A method of making a thermocouple comprising the steps of:

positioning a length of metallic wire inside a length of metallic tube in approximate axial alignment with the tube and set in from one end of the tube, the tube having an inside diameter appreciably larger than the diameter of the wire;

reducing the open area at the one end of the tube with a piece of metal dissimilar from the wire; and heat sealing the one end of the tube at the reduced area so the seal forms a thermocouple junction.

7. The method of claim 6, in which the open area at the one end of the tube is reduced by placing around the wire a sleeve made from a metal compatible with the tube, the sleeve extending beyond the set-in end of the wire and the hot thermocouple junction is formed between the sleeve and the portion of the wire it surrounds.

8. The method of claim 6, in which the open area at the one end of the tube is reduced by placing in the mouth of the tube in spaced relationship from the set-in end of the wire a pellet made from a metal compatible with the tube.

9. A method of making a thermocouple comprising the steps of:

increasing at one end the effective cross-sectional area of a length of metallic wire;

positioning the length of wire inside a length of metallic tube in approximate axial alignment with the tube, one end of the wire being set in from one end of the tube so the tube extends beyond the wire; and

heat sealing the one end of the tube from the atmosphere after positioning the length of wire so the seal forms a thermocouple junction.

10. The method of claim 9, in which the effective cross-sectional area of the one end of the wire is increased at the one end by upsetting and the sealing step forms a hot thermocouple junction between the upset one end of the wire and the one end of the tube by welding shut the portion of the tube extending beyond the wire.

11. The method of claim 9, in which the effective cross-sectional area of the one end of the wire is increased by placing a short tight fitting sleeve over the one end of the length of wire.

12. The method of claim 11, comprising the step of forming a hot thermocouple junction between the adajcent surfaces of the length of wire and the sleeve.

13. The method of claim 11, in which the sleeve is open at its end.

14. The method of claim 11, in which the sleeve is closed at its end.

15. A method of making a thermocouple comprising the steps of:

placing a short tight fitting metallic sleeve over one end of a length of metallic wire;

positioning the length of wire inside a length of metallic tube in approximate axial alignment with the tube such that the sleeve is located at one end of the tube, the tube being metallically compatible with the sleeve; and

heat sealing the one end of the tube from the atmosphere after the placing and positioning steps so the seal forms a thermocouple junction.

16. The method of claim 15, additionally comprising the step of forming a hot thermocouple junction at the adjacent surfaces of the sleeve and the wire.

17. The method of claim 16, in which the sleeve, the tube, and the wire are made from dissimilar metallic compositions.

18. A method of making a thermocouple comprising the steps of:

reducing at one end a length of tube made from one metal;

positioning a length of wire made from a dissimilar metal inside the tube in axial alignment with the tube, one end of the wire being set in from the one end of the tube so the tube extends slightly beyond the wire but the reduced portion of tube is adjacent to the one end of the wire; and

heat sealing the one end of the tube from the atmosphere adjacent the one end of the wire by welding shut the portion of the tube extending beyond the wire so the seal forms a hot thermocouple junction between the one end of the wire and the tube.

19. The method of claim 15, additionally comprising the step of forming a hot thermocouple junction at the adjacent surfaces of the sleeve and the tube.

P0-1050 I UNITED STATES PATENT OFFICE 569 CERTIFICATE OF CORRECTION PatentNo. 3 7 7I ZQZ Dated N h 21 1 Inventor(s) Ed Iond M, wggg .I

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

f" "I Patent co1umn 6,- line 67, change "maing" to --making--'-.

Patent column 7 line 9, after "tube" insert -'--after reducing the open area--;

I line 1*], change "pleeet" to --pellet-.

Signed and sealed this 11th day of June 19714..

(SEAL) Attest:

EDWARD .M.RLET0HER,JR. c. MARSHALL 1mm Attesting Officer. Comiasioner of Patents g gg OUINIITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3 Invent0r(s) Edmond M. Wagner Y It is certified that: error appears invthe above-identified patent and that said Letters Patent are hereby corrected as shown below:

Patent column- 6, line 67, change "maing" to --making--'-.

Patent column 7, line 9, after ."tube" insert-after reducing the open area--;

'line 11, change "pleeet" to --pe11et--.

Signed and sealed this 11th day of June 1971 (SEAL) 'Attest:

EDWARD-M-FLETCHERJR; I c. MARSHALL pm Attesting Officer v Commissioner of Patents 

1. The method of maing a thermocouple comprising the steps of: positioning a length of wire made from one metal inside a length of tube made from a dissimilar metal in approximate axial alignment with the tube and set in from one end of the tube, the tube having an inside diameter appreciably larger than the diameter of the wire; reducing the open area at the one end of the tube after positioning the wire inside the tube; applying heat to the one end of the tube to seal the one end of the tube from the atmosphere at the reduced area so the seal forms a hot thermocouple junction between the tube and the wire; and forming a cold thermocouple junction including at least one of the lengths.
 2. The method of claim 1, in which the open area at the one end of the tube is reduced by placing in the mouth of the tube on the set-in end of the wire a pleeet made from a metal compatible with the tube.
 3. The method of claim 1, in which the open area at the one end of the tube is reduced by swaging a portion of the length of the tube at the one end onto the adjacent portion of the wire.
 4. The method of claim 3, in which the open area at the one end of the tube is further reduced by placing in the mouth of the tube on the set-in end of the wire a pellet made from a metal compatible with the tube.
 5. The method of claim 3, in which the open area at the one end of the tube is further reduced by bending in the portion of the length of tube extending beyond the set-in end of the wire at the one end of the tube.
 6. A method of making a thermocouple comprising the steps of: positioning a length of metallic wire inside a length of metallic tube in approximate axial alignment with the tube and set in from one end of the tube, the tube having an inside diameter appreciably larger than the diameter of the wire; reducing the open area at the one end of the tube with a piece of metal dissimilar from the wire; and heat sealing the one end of the tube at the Reduced area so the seal forms a thermocouple junction.
 7. The method of claim 6, in which the open area at the one end of the tube is reduced by placing around the wire a sleeve made from a metal compatible with the tube, the sleeve extending beyond the set-in end of the wire and the hot thermocouple junction is formed between the sleeve and the portion of the wire it surrounds.
 8. The method of claim 6, in which the open area at the one end of the tube is reduced by placing in the mouth of the tube in spaced relationship from the set-in end of the wire a pellet made from a metal compatible with the tube.
 9. A method of making a thermocouple comprising the steps of: increasing at one end the effective cross-sectional area of a length of metallic wire; positioning the length of wire inside a length of metallic tube in approximate axial alignment with the tube, one end of the wire being set in from one end of the tube so the tube extends beyond the wire; and heat sealing the one end of the tube from the atmosphere after positioning the length of wire so the seal forms a thermocouple junction.
 10. The method of claim 9, in which the effective cross-sectional area of the one end of the wire is increased at the one end by upsetting and the sealing step forms a hot thermocouple junction between the upset one end of the wire and the one end of the tube by welding shut the portion of the tube extending beyond the wire.
 11. The method of claim 9, in which the effective cross-sectional area of the one end of the wire is increased by placing a short tight fitting sleeve over the one end of the length of wire.
 12. The method of claim 11, comprising the step of forming a hot thermocouple junction between the adajcent surfaces of the length of wire and the sleeve.
 13. The method of claim 11, in which the sleeve is open at its end.
 14. The method of claim 11, in which the sleeve is closed at its end.
 15. A method of making a thermocouple comprising the steps of: placing a short tight fitting metallic sleeve over one end of a length of metallic wire; positioning the length of wire inside a length of metallic tube in approximate axial alignment with the tube such that the sleeve is located at one end of the tube, the tube being metallically compatible with the sleeve; and heat sealing the one end of the tube from the atmosphere after the placing and positioning steps so the seal forms a thermocouple junction.
 16. The method of claim 15, additionally comprising the step of forming a hot thermocouple junction at the adjacent surfaces of the sleeve and the wire.
 17. The method of claim 16, in which the sleeve, the tube, and the wire are made from dissimilar metallic compositions.
 18. A method of making a thermocouple comprising the steps of: reducing at one end a length of tube made from one metal; positioning a length of wire made from a dissimilar metal inside the tube in axial alignment with the tube, one end of the wire being set in from the one end of the tube so the tube extends slightly beyond the wire but the reduced portion of tube is adjacent to the one end of the wire; and heat sealing the one end of the tube from the atmosphere adjacent the one end of the wire by welding shut the portion of the tube extending beyond the wire so the seal forms a hot thermocouple junction between the one end of the wire and the tube.
 19. The method of claim 15, additionally comprising the step of forming a hot thermocouple junction at the adjacent surfaces of the sleeve and the tube. 